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Groundwater Availability in Central Maui, Hawaii

Project Chief: Stephen Gingerich
Project Period: March 2002 through September 2007
Cooperator: Maui County Department of Water Supply

Location

The main area of interest for this study is the central part of the island of Maui, which includes the Iao aquifer area and the central isthmus. However, the overall study area includes all of west Maui, the central isthmus, and about a third of east Maui to provide an overall understanding of the regional groundwater flow system.

Problem

Map of Maui showing location of study area

Between 1970 and 2000, the resident population on the island of Maui increased over 200 percent, and groundwater demand also increased significantly during this period. In response to increased withdrawals from the freshwater-lens system of the Iao aquifer, the most important aquifer in central Maui, water levels have declined, the transition zone between freshwater and saltwater has risen, and the chloride concentrations of water pumped from wells has increased. This has led to concern over the long-term sustainability of withdrawals from existing wells in the Iao aquifer. To ensure prudent management of the groundwater resources of the Iao aquifer and throughout central Maui, and to plan for sustainable growth on the island, an improved understanding of the groundwater flow system is needed. This study will provide information needed to better manage the groundwater resources of central Maui.

Objectives

The objectives of this 4 1/2-year study were to: (1) obtain a better understanding of the regional groundwater flow system in the study area, (2) estimate groundwater recharge in the study area, and (3) estimate the effects of selected withdrawal scenarios, using a numerical groundwater flow and transport model, on water levels, the transition zone between freshwater and saltwater, and surface-water/groundwater interactions in the main area of interest.

Relevance and Benefits

The results from this study are necessary for the optimal management of the ground and surface waters in Hawaii and, thus, the study is consistent with the mission of the USGS Strategic Plan to provide scientific information to manage water resources and to protect our quality of life. The mission of the Water Resources Discipline, which supports the overall mission of the USGS, is to provide the hydrologic information and understanding needed for the best use and management of the Nation's water resources for the benefit of the people of the United States. This study provides information needed for water-use permitting and land-use planning in an area that is rapidly being developed.

Approach

To meet the objectives of this study, a four-phased approach was used. The four phases included: (1) a review and analysis of existing data, and the development of a preliminary conceptual model of the groundwater flow system; (2) collection of climate, streamflow, and groundwater data; (3) a water-budget computation to estimate groundwater recharge rates; and (4) development of a refined conceptual model of the groundwater flow system and a numerical groundwater flow and transport model. Existing data will be compiled and analyzed to determine the relation between groundwater and surface water, regional groundwater flow patterns, and aquifer hydraulic characteristics, and to identify additional data needs. Additional data were collected to improve the understanding of climatic characteristics (rainfall, evapotranspiration, fog drip) and to fill other data needs identified in the first phase of the study. On the basis of existing data and climate data collected for this study, a water budget was computed to estimate groundwater recharge. The estimated recharge and conceptual model developed from this study was used to construct a regional numerical groundwater model. The numerical groundwater model was used to estimate the effects of selected withdrawal and recharge scenarios on groundwater availability.

Products

For this study, we compiled surface-water discharge measurements, water-level data, groundwater withdrawal and salinity measurements, aquifer tests, geologic logs, geophysical surveys from readily available sources, maintained climate stations at three sites (Puu Kukui, Kaulewelewe, and Kahakuloa), maintained water-level and/or pumping sensors in more than 20 wells, and conducted seepage runs on Waihee River and Iao, Waikapu, Waiehu, and Makamakaole Streams. We also worked with the National Geodetic Survey to verify benchmark elevations to ensure that well-level monitoring wells used in this study are tied to a common datum. Revised measuring-point elevations for selected wells in the Waihee and Iao aquifer areas were made available on January 6, 2004. On May 17, 2005, we collected nearly simultaneous water-level measurements from about 30 wells in central Maui with cooperation from the Maui DWS, HC&S, and other well owners. In 2007, we published Effects of Agricultural Land-Use Changes and Rainfall on Groundwater Recharge in Central and West Maui, Hawaii, 1926-2004. A numerical groundwater flow and transport model was constructed to evaluate groundwater pumping scenarios. In 2008, we published Ground-Water Availability in the Wailuku Area, Maui, Hawai'i and the study was concluded.

A liaison committee for the study was established that includes representatives from agencies and organizations that have water-resource responsibilities and interests. Committee activities included the exchange of information about water-resource issues, identification of sources of data and information, assistance in site selection and in the design and scope of study products, and the review of study results. Liaison committee meetings were held in February and September 2004, April 2005, March 2006, and August 2007.